1. Field of the Invention
The invention relates to a method for initializing filter coefficients, used in an echo-cancelling arrangement incorporated in a transceiver equipment to cancel an echo signal occurring in the receive path in response to a signal applied to the transmit path and consisting of a substantially undelayed near echo and a delayed distant echo, said echo-cancelling arrangement operating at a given sample rate and comprising a near echo canceller receiving a signal D subjected to the phase variations of the transmit carrier and a distant echo canceller receiving the said signal D subjected additionally to a delay which is substantially equal to the measured distant echo delay .tau. end echo, this method being intended for the initialization of the filter coefficients of the near and distant echo cancellers.
2. Description of the Prior Art
It is known that echo cancellers are adaptive devices which are formed with the aid of filters having adjustable coefficients and which are incorporated in data-transmission modems connected to a two-way transmission circuit in order to cancel automatically undesirable echoes occurring in the one-way receive path in response to the signal applied to the one-way transmit path. Conventional echo cancellers are generally designed to cancel echo signals which are not delayed or relatively little delayed, occurring on national and international terrestrial circuits.
However, international communications are being increasingly conducted via communication satellites. In a circuit of this kind, including a satellite link between two radio-relay stations, there may be produced in the receive path of a modem a near echo which is not or little delayed and is generated in the part of the circuit preceding satellite link, as well as a distant echo which is generated in the part of the circuit after the satellite link and which is therefore subject to a considerable delay .tau., depending particularly on the wave-propagation time in the satellite link. Since the satellite used may or may not be geostationary and since the terrestrial circuit may differ according to the connections, it can be estimated that in the international switched network the delay .tau. of the distant echo may assume values ranging between approximately 220 and 630 ms.
To cancel the echo signal consisting of a near echo and a distant echo, which each have a relatively short duration of the order of 10 ms or several tens of ms but which are separated by a large time interval of the order of the delay .tau., it is an advantage to use the echo-cancelling arrangement having the configuration described above and known from the article by Stephen B. Weinstein, entitled "A Passband Data Driven Echo Canceller for Full Duplex Transmission on Two-Wire Circuits", and published in the journal IEEE Transactions on Communications, Vol. COM-25, No. 7, July 1977, pp. 654-666. This configuration comprises an adaptive transversal filter, which receives a signal from the transmit path directly and which provides a copy of the near echo when its coefficients are suitably adjusted, and another adaptive transversal filter, which receives the signal from the transmit path subject to a delay equal to the measured distant echo delay and which delivers a copy of the distant echo when its coefficients are suitably adjusted. By subtracting from the received signal the sum of the signals leaving the two filters, the near and distant echoes in the receive path are cancelled. This configuration, which necessitates a, at least rough, previous measurement of the delay .tau. of the distant echo, has the advantage of using adaptive filters whose complexity is not unreasonable.
For adjustment of the coefficients of the two adaptive filters after measurement of the delay .tau. of the distant echo, the article by Weinstein referred to above suggests using the gradient algorithm, even during a training period preceding the full duplex transmission of the useful data. With this algorithm the coefficients are adjusted iteratively and tend asymptotically towards their optimum values, leading to a somewhat slow convergence of the two echo cancellers.